The present invention relates to a method for nondestructively determining the performance characteristics of a metal component. More specifically, the present invention relates to linking impact treatment process variables to surface texture and determining the impact process"" reliability and reproducibility of process variables linked to fatigue life, stress corrosion resistance enhancement, or other related performance parameters of an impact treated or shot peened metal component.
Impact treatment processes, including shot peening, have long been known to be capable of developing large fatigue strength and/or stress corrosion resistance enhancements in cyclically stressed metallic workpieces. In addition, such processes are known to be capable of generating certain workpiece surface morphology characteristics which are advantageous in certain applications.
Impact treatment processes used to improve workpiece performance include shot peening, sand blasting, laser shock processing and any other such processes which produce surface plastic deformation resulting from the surface being impacted with relatively high energy. Such impact treatment processes enhance the performance characteristics of a component through modification of a component""s surface and subsurface characteristics. During the shot peening process, a stream of shot or other near spherical particles typically made of a metal or glass travelling at high velocity is directed at a component surface. The shot is directed at the component so as to cause plastic deformation and strain of the component surface. Similarly, lasers which generate high energy are used to impact treat surfaces by generating plastic deformation and strain. Plastic strain is the mechanism where the surface of the component is put into a state of compressive residual stress, increased dislocation density, and other metallurgical microstructural changes, these being key to the life enhancing benefits of impact treatment processes. Depending on the nominal, range, and distribution of the values of certain critical impact treatment process variables or parameters such as in shot peening where shot size, shot velocity, shot impact angle, and workpiece shot peen process saturation are critical variables, the shot peen process induces different magnitudes and depths of residual compressive stresses and microstructural changes and varying degrees and types of surface morphology characteristic changes. In laser shock processing, variables such as laser intensity and the number of impacts or xe2x80x9cshotsxe2x80x9d in a given area determine the level of the desired workpiece characteristics. Each of these impact process induced surface and subsurface changes and enhancements are quantitatively and reliably related to impact process variable""s nominal, range, and distribution values for given workpieces of given chemical and physical characteristics. These relationships are well established in the art, particularly in shot peening.
Due to the shot peening processes total number of critical process parameters and the total number of ways changes in those variable values can be affected during processing, the shot peen process has developed a history of variability in process induced benefit when applied to workpieces in a production processing environment. This is true unless a very high degree of care is taken to positively and systematically control all shot peen process variables within relatively narrow tolerances.
The current best available method of defining what shot peen process should be applied to test workpieces and/or predicting whether a workpiece processed in a production application of shot peening would perform as well as expected is to determine the magnitude of the residual stresses introduced by the shot peen process, in profile and in depth, through x-ray diffractometer analysis, a technique well documented in the art. This is, however, a destructive inspection process as it requires extensive electrochemical milling of the surface being analyzed. It is also relatively costly and time consuming enough to be ineffective as a real time production inspection technique.
The standard industry accepted method of inspecting and calibrating a shot peen process involves the exposure of an SAE 1070 steel strip (Almen strip) to the blast stream of shot. This strip is mounted by four retaining screws on a thick block and placed in the blast in a manner which best represents the workpiece surface for the same exposure time the workpiece will experience. As an Almen strip is exposed to the blast stream, it arcs towards the stream. The amount of arc when stabilized over time (Almen saturation) indicates the total energy of the blast stream. The shot peen process cycle time is also typically determined by and/or calculated from the exposure time required to achieve Almen saturation utilizing Almen strips. This is a technique well established in the prior art.
While Almen intensity is an indicator of total blast stream energy, the method has significant shortcomings as a valid non-destructive inspection of the efficacy of a shot peen process in developing workpiece fatigue strength and/or other shot peen processes induced workpiece performance enhancements. First, the Almen strip is rarely of the same material or hardness as the workpiece. Second, it is only representative of the shape of a workpiece if that workpiece""s area being shot peened is essentially flat. As such, an Almen strip, which is intended to be only a representation of the workpiece being processed, is also typically a very poor representation of the workpiece. Additionally, the total cumulative inherent measurement system variability of the Almen measurement system as specified in industry and U.S. military specifications has been documented in the prior art as being large enough to often exceed the total allowable manufacturing process tolerances for the shot peen process it is attempting to calibrate and measure.
While there are several other serious drawbacks and shortcomings to the Almen system of shot peen process measurement, which are well documented in the art, perhaps most importantly the height of an Almen strip at Almen saturation (Almen intensity) measures only the aggregate energy transfer to the Almen strip by the shot peen process in the form of residual compression. This aggregate residual compression is the aggregate effect of the nominal, range, and distribution values of all of a shot peen processes variables experienced during the processing of the Almen strip. An Almen intensity value as such cannot indicate what the nominal residual compression is at any given point on the surface and/or subsurface respectively. This effectively precludes the Almen system from measuring the individual level and effect of any or all shot peen process parameters. Thus, process parameters such as shot size, shot velocity, and shot angle of impact, cannot be predicted individually or quantitatively predicted by the Almen technique without unequivocally knowing the quantitative value of the other process parameters and the quantitative level of variability and measurement caused by cumulative measurement of variability. For example, one may deliver the same aggregate amount of energy to the workpiece by using large shot at a small velocity or small shot at a high velocity. In either case, the Almen strip can indicate the same effective measurement. As such, a given Almen intensity value can be arrived at through an infinite combination of process parameter nominal and range values, each combination having it""s own unique pattern of residual stress magnitude in depth and in profile. Since the magnitude and depth of residual compression and other shot peen process induced workpiece changes are highly related to workpiece fatigue strength and other shot peen process induced workpiece benefits, the Almen measurement system cannot be counted on to quantitatively correlate a shot peen process with the desired level of benefits it is intended to induce in the workpieces. This phenomena is well established in the prior art.
For a given workpiece of given chemical and physical characteristics, a given shot particle of given size impacts a metallic workpiece surface at a specific impact angle and velocity, leaving a unique indentation mark or xe2x80x9cfingerprintxe2x80x9d. As multiple shot particles impact the same general area, the workpiece surface in that area becomes increasingly covered with indentations. As subsequent shot impact indentations overlap previous indentations, some of the workpiece surface morphology characteristics of shot impact associated with shot of given shot peen process variable levels change. However these changes are reliably predictable for given workpiece saturation levels.
With statistically capable controls on key variables such as shot size and velocity, the variability from a peening operation in terms of results such as fatigue strength enhancement and surface texture development may be reduced. However, without destructively testing the workpiece, when inspecting a shotpeened workpiece, one is never totally assured of the stability of shot peen process induced fatigue strength and/or stress corrosion resistance improvements offered by the peening process the workpiece experienced. It is never totally assured that critical process parameter levels did not vary unacceptably during processing. The ability to assure that a surface has been properly treated by a shot-peening process relies on statistical controls of all peening variables. However, there is no absolute certainty that the xe2x80x9ceffectivexe2x80x9d parameter (e.g. shot velocity) as measured by a sensor and controller is that which the workpiece actually experienced. Accordingly, a technique is needed to assess impact treated or shot peened workpieces to assure that the desired peening conditions have all been met and thus the fatigue and corrosion resistance advantages expected are present in the workpiece being assessed.
The present invention is the first effective nondestructive inspection (NDI) technique for the analysis of an impact treated or shot peened surface. The basis of this NDI technique is the assumption that after being impacted, a surface contains the xe2x80x9cfingerprintxe2x80x9d of the impact process and/or individual impacts in the generated surface texture. For example, a surface shot peened with smaller diameter shot will tend to have smaller diameter craters as compared to a surface peened with larger shot. Thus, the NDI technique requires the ability to discern quantitatively, those aspects of the surface texture that may be best related to the impact parameters.
While shot peening is the primary method of surface treatment analyzed in the present invention to improve the performance of metal workpieces, alternate surface treatment methods, such as laser or other electromagnetic surface treatments, exist to which the present invention may be applied. In laser surface treatment, a laser is directed at the surface of a workpiece typically covered with a black paint, causing a high energy shock wave to be directed into the workpiece generating plastic deformation and residual compressive stresses in the surface and subsurface of the workpiece, similar qualitatively to the effect of shot peening.
With the advent of the powerful desk top computer and various advanced analytical software packages, it is now possible to apply an intense amount of analysis to the impact treatment process. The present invention provides an NDI technique that is capable of predicting the impact treatment process variables experienced on an impact processed surface or workpiece. From such information, provided that performance enhancement characteristics are known for a given impact process of a given parameter values and tolerances for the workpiece in question, the presence or absence of such performance characteristics as increased fatigue life and stress corrosion resistance enhancement can be identified at the workpiece level. The present invention comprises a method and apparatus to analyze the surface texture characteristics of an impact treated component to determine and predict the quantitative level of impact treatment process parameters experienced on the analyzed surface or workpiece during its impact processing.
Through development of a data base that quantitatively relates shot peen process variable levels to certain workpiece chemical and physical characteristics, and the resultant residual stresses in profile, surface morphology characteristics of a shot peened workpiece of known chemical and physical characteristics can reliably predict the magnitude of residual stresses in profile and in depth induced by the shot peen process the workpiece experienced. For example, after the process parameter values are determined on an impact treated workpiece, empirical data in the database linking the process variables to performance critical characteristics such as residual stress can be used based on established test data to predict the actual performance characteristics of the analyzed workpiece.
This new NDI technique takes advantage of developments in high speed two dimensional (2D) and three dimensional (3D) non-contact surface profilometry to provide detailed images of the impact treated or peened surfaces. The NDI approach incorporates statistical analysis such as designed experiments (DOE), correlation analysis, principal component analysis, and multiple regression (MREG), all available in common commercial software. The present invention is the first effective NDI technique for the analysis of an impact treated or shot peened surface.
In summary, a first embodiment of the invention may be described as a nondestructive method for the inspection of impact treated surfaces to predict impact treatment process parameter levels and tolerances experienced during impact treatment processing on the workpiece being inspected comprising: acquiring a multi-dimensional map of an impact treated surface, characterizing the multi-dimensional map to develop surface texture parameters, and developing relationships between the surface texture parameters and the quantitative impact treatment process parameter levels experienced. In a second embodiment, the invention may be described as an apparatus for determining the impact treatment process parameter levels of an impact treated surface comprising: an imaging system which captures the surface texture of the impact treated surface in a multi-dimensional image, a processor which analyzes the multi-dimensional image to isolate a plurality of statistically independent variables, where the statistically independent variables describe the surface texture of the impact treated surface and are used to predict the impact treatment process parameter levels and tolerances.
Further objects, features, and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.